The invention relates to fiber-optic sensors, in particular fiber-optic gyroscopes and fiber-optic sensors that measure magnetic fields.
Fiber-optic sensors can be used to measure various physical quantities, such as the rotation of an object (fiber-optic gyroscope) or magnetic fields arising from currents in the vicinity of the sensor (fiber-optic current sensor). Such fiber-optic sensors typically include a section of optical fiber that is coupled to a broadband source, such as a laser operated below threshold, a superluminescent diode (SLD) or a fiber superluminescent source, a coupler to couple optical radiation emitted by the broadband source into the fiber, preferably a fiber coil, at least one polarizer and at least one phase modulator, and a detector which detects a phase shift or polarization difference between the optical signals traversing the fiber. The phase or polarization shift between the optical signals may be introduced, for example, by the rotation of the fiber coil or by a magnetic field. The term fiber-optic sensor will be used hereinafter to refer to both fiber-optic gyroscopes and fiber-optic current sensors.
The electrical output signal of a fiber-optic sensor includes noise components arising from various sources. The output noise which tends to manifest itself as angle random walk (ARW), as defined, for example, in IEEE Std-528, Inertial Sensor Terminology (incorporated by reference herein), has a different functional dependence on the optical power at the photodetector input, depending on the origin of the noise component. For example, thermal noise generated in the transimpedance amplifier feedback resistor in the photodetector electronics is independent of the light power. Shot noise related to the quantized nature of the detector current can be shown to be proportional to the square root of optical power, whereas relative intensity noise (RIN), which is inherent in the light of the source due to its finite bandwidth and impinges on the photodetector, and flicker noise (1/f), are proportional to the optical power. Since RIN and flicker noise have the same functional dependence on optical power, the term RIN hereinafter refers to both RIN and flicker noise.
RIN causes the noise-related performance of fiber-optic sensor systems to saturate, rather than continue to improve, as the source power is increased. Unless RIN can be mitigated, there exists a power level beyond which no further practical improvement is possible. For example, EDFS (Erbium doped fiber sources) which, due to their high optical power and wavelength stability, are often considered the natural choice for high performance fiber-optic sensors, tend to have a high RIN. Superluminescent diodes (SLD""s), on the other hand, tend to have a lower RIN due to their larger bandwidth, but may suffer from limitations in power and lifetime, limiting their utility.
In one approach described in U.S. Pat. No. 5,331,404 and illustrated in FIG. 1, RIN in fiber-optic sensors is reduced by coupling a fraction of the light emitted by a light source 19 into a xe2x80x9cdummyxe2x80x9d fiber 30 having substantially the same length as the fiber 22 of the fiber-optic sensor 5. The output signal detected at the end of the xe2x80x9cdummyxe2x80x9d fiber 30 by detector 34 is then modulated in multiplier 36 by a replica of the signal output of the fiber optic sensor 5 detected by detector 32 and subtracted in subtractor 38 from the output signal of the fiber-optic sensor 5 detected by detector 32 after passing through AC coupled amplifiers 40, 42 with suitable adjustment of the channel gains. In other words, the xe2x80x9cdummyxe2x80x9d fiber 30 in this case operates as an analog delay line to match the time delay experienced by the light traversing the fiber-optic sensor 5. This approach, however, requires a second coil of fiber of approximately the same length as the fiber-optic sensor coil.
In another approach disclosed in U.S. Pat. No. 5,655,035, two fiber-optic sensors can be excited by the same optical source, but with the sensitive axes oriented in diametrically opposed directions. The detected outputs are added, thereby subtracting the RIN, which is common to both channels since it arises in the common source. This approach doubles the entire fiber-optic sensor optical component count (except for the light source), which is expensive and bulky.
In another approach, described in U.S. Pat. No. 6,370,289 issued to Bennett, light from a light source is coupled into an input coupler with a first portion of the light emerging from the input coupler being transmitted to and through a first polarizer and a sensor which contains a fiber coil and other appropriate components. A detector and amplifier is coupled to a return tap of the input coupler and measures a sensor signal after the first portion of the light has transited through the sensor coil and other components. The sensor signal includes, in addition to the desired sensor signal, among others, the RIN noise. A second portion of the light emerging from the unused tap of the input coupler is transmitted through a second polarizer having a polarization axis substantially parallel to that of the first polarizer and is detected by a second detector and amplifier. This detected second portion of the light represents the RIN noise (as well as other incoherent noise sources). The RIN noise sample is delayed in a delay unit whose bandwidth is larger than the detector bandwidth, so that the time delay is essentially constant across the bandwidth where noise cancellation is desired and the sensor signal is present. The delay unit can be either analog delay line or implemented digitally in a shift register or in a computer memory buffer. The RIN noise sample is then multiplied by the waveform of the sensor signal. The sensor signal can also be passed through a DC block to eliminate the DC component of the sensor signal prior to the subtraction. The multiplied RIN noise sample is then subtracted from the sensor signal. The resulting time-dependent waveform having a reduced RIN component can then be processed further. According to another embodiment of the Bennett Patent, the RIN sample signal may be sampled at a rear facet of the light source.
The Bennett Patent approach, while having many advantages over the prior art, requires the use of a delay unit which can be, e.g., either analog delay line or implemented digitally in a shift register or in a computer memory buffer. It may be advantageous, however, to avoid the use of such components and implement the RIN noise reduction using other means to delay the RIN noise signal.
In addition, P. Polynkin, J. deArruda and J. Blake in xe2x80x9cAll-optical noise-subtraction scheme for a fiber optic gyroscopexe2x80x9d in Opt. Lett, Vol. 25, No. 3, pp-147-149, Feb. 1, 2000 describe a method which eliminates the need to delay the source signal, and performs the noise cancellation optically. They have shown that the source noise signal and the noise on the sensor signal have the same form at the optical detector for a closed-loop gyro with square wave modulation(except that they are out of phase), if the gains (losses) in each path are the same and the modulation frequency is the so-called xe2x80x9cproper frequencyxe2x80x9d, which is equal to
fp=1/(2xcfx84)
where xcfx84 is the transit time of the light though the sensor. The optical signals are imaged on the same detector with the resultant noise cancellation occurring in the detection process.
However, there are some practical difficulties with this technique, as the relative amplitude of the two optical signals is fixed at the time of manufacture, and it is known that the loss in the optical circuit changes with temperature. Also, since there is no provision for multiplying the source signal by the replica of the sensor signal, this method is only appropriate for closed-loop gyros where the output signal is nulled. In addition, the approach only works if the modulation frequency is the proper frequency.
The latter restriction is certainly a limitation for open-loop gyros where the modulation frequency is often a small fraction of the proper frequency.
It would therefore be desirable to provide a fiber-optic sensor that used a filter to delay the RIN noise signal.
The invention is directed to an apparatus and a method for reducing noise, in particular RIN noise, in fiber-optic sensors.
An optical sensor system and method for producing a sensor signal having reduced noise comprises an optical sensor adapted to receive a sensor input light and adapted to measure a physical quantity, with the optical sensor producing a sensor output signal corresponding to the physical quantity and a first noise component, a first detector which detects the sensor output signal, a second detector which detects the sensor input light corresponding to a second noise component, a high pass filter or DC block coupled to the second detector, a filter with a group delay substantially corresponding to a time delay of the sensor input light traversing the optical sensor coupled to the high pass filter or DC block, a multiplier, which multiplies the filtered time-delayed second noise component with the sensor output signal, and a subtractor which subtracts the multiplied filtered time-delayed second noise component from the sensor output signal to produce the sensor signal having the reduced noise. The optical sensor may comprise an optical waveguide, which may be an integrated optical waveguide. The optical sensor may be a fiber-optic sensor. The physical quantity may be a rotation of the fiber-optic sensor or a magnetic field. The magnetic field may be produced by an electric current. An amplifier may adjust an amplitude and optionally a phase of the filtered time-delayed second noise component relative to a respective amplitude and optionally a phase of the sensor output signal. The sensor output signal may be amplified so as to suppress a DC component of the sensor output signal after the filtered time delayed second noise component is multiplied by the sensor output signal, and before the multiplied filtered time-delayed second noise component is subtracted from the sensor output signal. The second detector may be coupled to an input section of the fiber-optic sensor or to a rear facet of the light source.
Further features and advantages of the present invention will be apparent from the following description of certain embodiments and from the claims.